Week 7
ATOMIC SPECTROSCOPY
Part 2 -
Atomic Emission & Fluorescence Spectroscopy
by Tan Ching Mun (16002662)
Atomic Emission Spectroscopy (AES)
-chemical analysis
-uses the intensity of light emitted from plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample.
AES requires no light source
Area of interest: Emission spectrum
-chemical analysis
-uses the intensity of light emitted from plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample.
AES requires no light source
Area of interest: Emission spectrum
2 types of AES
i) Based on Arc and Spark
-based on the excitation of emission spectra of elements with electric arcs or high voltage sparks (Temp=3000-6000K)
-An electric arc or spark is passed through the sample - heating it to a high temperature to excite the atoms within it
-The excited analyte atoms emit light at characteristic wavelengths that can be dispersed with a monochromator and detected
-An electric arc or spark is passed through the sample - heating it to a high temperature to excite the atoms within it
-The excited analyte atoms emit light at characteristic wavelengths that can be dispersed with a monochromator and detected
ii) Based on Plasma Source
-Most common - Inductively Coupled Plasma (ICP AES)
-Plasma is a gas that is hot enough to contain ions and free electrons
-Temperature up to 8000 K - high enough to excite most elements
-Most common - Inductively Coupled Plasma (ICP AES)
-Plasma is a gas that is hot enough to contain ions and free electrons
-Temperature up to 8000 K - high enough to excite most elements
Advantages
-Lower susceptibility to chemical interferences.
-Good emission spectra (multi-element analysis for a small sample)
-Energetic plasma source (for determination of low concentration of element and non-metal such as CI, Br, I and S.)
Disadvantages
-Complex and numerous lines – increase the probability of spectral interferences in quantitative analysis
-Require higher resolution & more expensive optical equipment
-Lower susceptibility to chemical interferences.
-Good emission spectra (multi-element analysis for a small sample)
-Energetic plasma source (for determination of low concentration of element and non-metal such as CI, Br, I and S.)
Disadvantages
-Complex and numerous lines – increase the probability of spectral interferences in quantitative analysis
-Require higher resolution & more expensive optical equipment
Therefore, AAS is more frequently used
-simpler
-less expensive
-lower operating cost
-greater precision
-require less operator
Atomic Fluorescence Spectroscopy (AFS)
- combination of AAS and AES
- the excitation from the ground state to the upper state is carried out radiationally (as in AAS)
- the measurement is made by detection of radiation emitted by the atom as it relaxes back from the excited state to the ground state (as in AES)
- combination of AAS and AES
- the excitation from the ground state to the upper state is carried out radiationally (as in AAS)
- the measurement is made by detection of radiation emitted by the atom as it relaxes back from the excited state to the ground state (as in AES)
Advantages
- Lower detection limit
- Greater sensitivity (fluorescence signal has very low background)
Disadvantages
- High cost & operational complexity
- Great success and widespread application in AAS - discourage further development in AFS.
Detection limit for Atomic Spectroscopy
Application of Atomic Spectrometry
- widely used in labs – trace element analyses
- Environmental samples for heavy-metal contamination
- Pharmaceutical samples - analyzed for metal impurities
- Steel industry - determine minor and major components
- sensitivity required
- number of samples
- single- or multi-element measurements
Reflection
We have come to the end of Chapter 5 Atomic Spectroscopy. After discussing this topic for two sessions, I can differentiate the 3 types of atomic spectroscopy, namely AAS, AES and AFS. AAS is mainly used due to several reasons such as simpler, cheaper device, lower operating cost and greater precision. Not only that, I also learned to apply the Beer’s Law to find the concentration of unknown sample through calibration curve. The lecture is delivered clearly with the aid of clear explanation and sample exercises.
Tutorial 4
2)What are the differences between a photon detector and a thermal detector?
A photon detector produces a current or voltage as a result of the emission of electrons from a photosensitive surface when struck by photons.
A thermal detector consists of a darkened surface to absorb infrared radiation and produce a temperature increase. A thermal transducer produces an electrical signal whose magnitude is related to the temperature and thus the intensity of the infrared radiations.
3) Define
(a) transducer.
Transducer indicates the type of detector that converts quantities, such as light intensity, pH, mass, and temperature, into electrical signals that can be subsequently amplified, manipulated, and finally converted into numbers proportional to the magnitude of the original quantity.
(b) n-type semiconductor
A semiconductor containing unbonded electrons (e.g. produced by doping silicon with a Group V element) is termed an n-type semiconductor.
(c) depletion layer.
A depletion layer results when a reverse bias is applied to a pn-junction type device. Majority carriers are drawn away from the junction leaving a non-conductive depletion layer.
4) Describe the differences between the following pairs of terms and list any advantages of one over the other:
(a) spectrophotometers and photometers.
Spectrophotometers have monochromators for multiple wavelength operation and for procuring spectra while photometers utilize filters for fixed wavelength operation. While offering the advantage of multiple wavelength operation, spectrophotometers are substantially more complex and more expensive than photometers.
(b) monochromators and polychromators.
Both a monochromator and a polychromator use a diffraction grating to disperse the spectrum, but a monochromator contains only one exit slit and detector while the polychromator contains multiple exit slits and detectors. A monochromator can be used to monitor one wavelength at a time while a polychromator can monitor several discrete wavelengths simultaneously.
5) What is the purpose of
(a) the 0% T adjustment
The 0% Transmittance is measured with no light reaching the detector and is a measure of the dark current.
(b) the 100% T adjustment of a spectrophotometer?
The 100% transmittance adjustment is made with a blank in the light path and measures the unattenuated source. It compensates for any absorption or reflection losses in the cell and optics.
6. Explain the mechanism of operation of a hollow-cathode lamp.
When a discharge occurs between two electrodes via a gas at low pressure, the cathode is bombarded by the energetic, positively-charged gas ions ( for example, ionized filler gas atoms) which are accelerated towards its surface by the potential existing in the discharge. The energy of these ions is such that atoms of the cathode material are ejected or “sputtered” into the plasma. Here they may collide with other high energy particles that are present. These collisions result in a transfer of energy causing the metal atoms to become excited. Since this excited state is not stable, the atoms relax back to their ground state, emitting radiation at the characteristic wavelength of that element. For most elements, more than one analytically useful spectral line is generated.
8) A municipal water company was having problems with its water analysis. The problem lay in the AAS determination of Fe at 248.3 nm. The absorbance of the water, after 5 fold dilution was 0.646 at 248.3 nm. A standard solution prepared by dissolving 0.1483g of iron wire in acid, diluting to 250 mL. After a further ×100 dilution the solution had an absorbance of 0.813. Calculate the ppm in the water sample. What is wrong with this analysis? On a proper investigation of the above problem a series of
iron standards were prepared by taking various volumes of a 0.0593 mg Fe per mL and
diluting up to 100 mL. The absorbance of the solutions were as follows:
1)A photometer with a linear response to radiation gave a reading of 625 mV with a blank in the light path and 149 mV when the blank was replaced by an absorbing solution.
Calculate
(a) The percent transmittance and absorbance of the absorbing solution.
(b) The percent transmittance to be expected if the light path through the original solution is doubled.
Calculate
(a) The percent transmittance and absorbance of the absorbing solution.
(b) The percent transmittance to be expected if the light path through the original solution is doubled.
2)What are the differences between a photon detector and a thermal detector?
A photon detector produces a current or voltage as a result of the emission of electrons from a photosensitive surface when struck by photons.
A thermal detector consists of a darkened surface to absorb infrared radiation and produce a temperature increase. A thermal transducer produces an electrical signal whose magnitude is related to the temperature and thus the intensity of the infrared radiations.
3) Define
(a) transducer.
Transducer indicates the type of detector that converts quantities, such as light intensity, pH, mass, and temperature, into electrical signals that can be subsequently amplified, manipulated, and finally converted into numbers proportional to the magnitude of the original quantity.
(b) n-type semiconductor
A semiconductor containing unbonded electrons (e.g. produced by doping silicon with a Group V element) is termed an n-type semiconductor.
(c) depletion layer.
A depletion layer results when a reverse bias is applied to a pn-junction type device. Majority carriers are drawn away from the junction leaving a non-conductive depletion layer.
4) Describe the differences between the following pairs of terms and list any advantages of one over the other:
(a) spectrophotometers and photometers.
Spectrophotometers have monochromators for multiple wavelength operation and for procuring spectra while photometers utilize filters for fixed wavelength operation. While offering the advantage of multiple wavelength operation, spectrophotometers are substantially more complex and more expensive than photometers.
(b) monochromators and polychromators.
Both a monochromator and a polychromator use a diffraction grating to disperse the spectrum, but a monochromator contains only one exit slit and detector while the polychromator contains multiple exit slits and detectors. A monochromator can be used to monitor one wavelength at a time while a polychromator can monitor several discrete wavelengths simultaneously.
5) What is the purpose of
(a) the 0% T adjustment
The 0% Transmittance is measured with no light reaching the detector and is a measure of the dark current.
(b) the 100% T adjustment of a spectrophotometer?
The 100% transmittance adjustment is made with a blank in the light path and measures the unattenuated source. It compensates for any absorption or reflection losses in the cell and optics.
6. Explain the mechanism of operation of a hollow-cathode lamp.
When a discharge occurs between two electrodes via a gas at low pressure, the cathode is bombarded by the energetic, positively-charged gas ions ( for example, ionized filler gas atoms) which are accelerated towards its surface by the potential existing in the discharge. The energy of these ions is such that atoms of the cathode material are ejected or “sputtered” into the plasma. Here they may collide with other high energy particles that are present. These collisions result in a transfer of energy causing the metal atoms to become excited. Since this excited state is not stable, the atoms relax back to their ground state, emitting radiation at the characteristic wavelength of that element. For most elements, more than one analytically useful spectral line is generated.
7) Compare the flame emission and atomic absorption spectroscopy with respect to instrumentation, sensitivity, and interferences.
8) A municipal water company was having problems with its water analysis. The problem lay in the AAS determination of Fe at 248.3 nm. The absorbance of the water, after 5 fold dilution was 0.646 at 248.3 nm. A standard solution prepared by dissolving 0.1483g of iron wire in acid, diluting to 250 mL. After a further ×100 dilution the solution had an absorbance of 0.813. Calculate the ppm in the water sample. What is wrong with this analysis? On a proper investigation of the above problem a series of
iron standards were prepared by taking various volumes of a 0.0593 mg Fe per mL and
diluting up to 100 mL. The absorbance of the solutions were as follows:
-What is the true value of the Fe content in the municipals water (use absorbance in part 1)?
-Calculate the percent error between the results.
-What assumption was made in part one that was not valid?
-Calculate the percent error between the results.
-What assumption was made in part one that was not valid?
